The present invention relates in general to the treatment of selenium alloy particles prior to vapor deposition of the selenium alloy on a substrate and using the treated selenium alloy particles in a process to vapor deposit a selenium alloy layer onto a substrate to form electrophotographic imaging members. One embodiment of the present invention is directed to a process which comprises providing an alloy of selenium, preparing powdered particles of the alloy with an average particle diameter of less than 300 microns, placing the powdered particles into a container and tumbling the container, and subsequently removing the powdered particles from the container and compressing the powdered particles into pellets. The present invention enables reduced fractionation of selenium and other alloying components during vacuum evaporation of selenium alloy particles that have been subjected to the process.
The formation and development of images on the imaging surfaces of electrophotographic imaging members by electrostatic means is well known. One of the most widely used processes is xerography, described in, for example, U.S. Pat. No. 2,297,691 to Chester Carlson. Numerous different types of electrophotographic imaging members for xerography, i.e. photoreceptors, can be used in the electrophotographic imaging process. Such electrophotographic imaging members can include inorganic materials such as selenium and selenium alloys, organic materials, and mixtures thereof. Electrophotographic imaging members can comprise contiguous layers in which at least one of the layers performs a charge generation function and another layer forms a charge carrier transport function, or can comprise a single layer which performs both the generation and transport functions. These electrophotographic imaging members can also be coated with a protective overcoating to improve wear.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed, and improve color copyability. These devices are typically based on alloys of selenium with tellurium and/or arsenic. The selenium electrophotographic imaging members can be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic, or selenium-tellurium-arsenic alloy layer which performs both charge generation and charge transport functions. The selenium electrophotographic imaging members can also comprise multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium alloy generator layer.
A common technique for manufacturing photoreceptor plates entails vacuum deposition of a selenium alloy to form an electrophotographic imaging layer on a substrate. Tellurium is incorporated as an additive for the purpose of enhancing the spectral sensitivity of the photoconductor. Arsenic is incorporated as an additive for the purpose of improving wear characteristics, passivating against crystallization, and improving electricals. Typically, the tellurium addition is incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy base layer in order to achieve the benefits of the photogeneration characteristics of SeTe with the beneficial transport characteristics of SeAs alloys.
Fractionation of the tellurium and/or arsenic composition during evaporation results in a concentration gradient in the deposited selenium alloy layer during vacuum evaporation. Thus, the term "fractionation" is used to describe inhomogeneities in the stoichiometry of vacuum deposited alloy thin films. Fractionation occurs as a result of differences in the partial vapor pressure of the molecular species present over the solid and liquid phases of binary, ternary, and other multicomponent alloys. Alloy fractionation is a generic problem with chalcogenide alloys. A key element in the fabrication of doped photoreceptors is the control of fractionation of alloy components such as tellurium and/or arsenic during the evaporation of selenium alloy layers. Tellurium and/or arsenic fractionation control is particularly important because the local tellurium and/or arsenic concentration at the extreme top surface of the structure, denoted as top surface tellurium (TST) or top surface arsenic (TSA), directly affects xerographic sensitivity, charge acceptance, dark discharge, copy quality, photoreceptor wear, and crystallization resistance. In deposited layers of alloys of Se-Te, the normal percentages of top surface tellurium can cause excessively high photosensitivity. This photosensitivity is variable and changes as the surface of the layer wears away. Surface injection of corona deposited charge and thermally enhanced bulk dark decay involving carrier generation cause the toner images in the final copies to exhibit a washed out, low density apperance. Excessive dark decay causes loss of high density in solid areas of toner images and general loss of image density. In single layer low arsenic selenium alloy photoreceptors, arsenic enrichment at the top surface due to fractionation can also cause severe reticulation of the evaporated film, electrical instability, and excessive dark decay. In two layer of multilayer photoreceptors where low arsenic alloys may be incorporated as a base or transport layer, arsenic enrichment at the interface with the layer above can lead to severe residual cycle up problems. In single layer tellurium selenium alloy photoreceptors, tellurium enrichment at the top surface due to fractionation can cause undue sensitivity enhancement, poor charge acceptance and enhancement of dark discharge. In two layer or multilayer photoreceptors where tellurium alloys may be incorporated as a generator layer, tellurium enrichment at the upper surface of the tellurium alloy layer can result in similar undue sensitivity enhancement, poor charge acceptance, and enhancement of dark discharge.
Thus, a significant problem encountered in the fabrication of selenium alloy photoreceptors is the fractionation or preferential evaporation of a species such that the resulting film composition does not replicate the original composition. In other words, the deposited film or layer does not have a uniform composition extending from one surface to the other. For example, when tellurium is the dopant, the tellurium concentration is unduly high at the top surface and approaches zero at the bottom of the vacuum deposited layer. This problem is also observed for alloys of Se-Te, Se-As, Se-As-Te, Se-As-Te-Cl, and the like and mixtures thereof.
U.S. Pat. No. 4,822,712 (Foley et al.), the disclosure of which is totally incorporated herein by reference, discloses an alloy treatment process which comprises providing particles of an alloy comprising amorphous selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, the particles having an average particle size of at least about 300 microns and an average weight of less than about 1000 milligrams, forming crystalline nuclei on at least the surface of the particles while maintaining the substantial surface integrity of the particles (by processes which may include mechanical abrasion of the particles), heating the particles to an initial temperature between about 50.degree. C. and about 80.degree. C. for at least about 30 minutes to form a thin, substantially continuous layer of crystalline material on the surface of the particles while maintaining the core of selenium alloy in the particles in an amorphous state, and rapidly heating the particles to at least a second temperature below the softening temperature of the particles that is at least 20.degree. C. higher than the initial temperature and between about 85.degree. C. and about 130.degree. C. to crystallize about 5 to 100 percent by weight of the amorphous core of selenium alloy in the particles while maintaining the integrity of the alloy particles and inhibiting the loss of selenium rich material. The resulting crystallized particles in shot or pellet form may be rapidly heated in a vacuum chamber to vacuum deposit the alloy onto a substrate.
U.S. Pat. No. 4,842,973 (Badesha et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for fabricating an electrophotographic imaging member which comprises providing in a vacuum chamber at least one crucible containing particles of an alloy comprising selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, providing a substrate in the vacuum chamber, applying a partial vacuum to the vacuum chamber, and rapidly heating the crucible to a temperature between about 250.degree. C. and 450.degree. C. to deposit a thin continuous selenium alloy layer on the substrate. A plurality of selenium containing layers may be formed by providing in a vacuum chamber at least one first layer crucible containing particles of selenium or a selenium alloy, at least one second layer crucible containing particles of an alloy comprising selenium, and a substrate, applying a partial vacuum to the vacuum chamber, heating the particles in the first layer crucible layer to deposit a thin continuous selenium or selenium alloy first layer on the substrate, maintaining the particles in the second layer crucible at a first temperature below about 130.degree. C. while the thin continuous selenium or selenium alloy first layer is deposited on the substrate, and rapidly heating the particles in the second layer crucible to a second temperature between about 250.degree. C. and about 450.degree. C. to deposit a thin continuous selenium alloy second layer on the substrate. The selenium alloy shot or pellet particles employed may be subject to surface abrasion prior to evaporation to produce surface crystallization.
U.S. Pat. No. 4,780,386 (Hordon et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for preparing an electrophotographic imaging member comprising providing large particles of an alloy comprising selenium, tellurium and arsenic, the large particles having an average particle size of at least about 300 microns and an average weight of less than about 1000 milligrams, mechanically abrading the surfaces of the large particles while maintaining the substantial surface integrity of the large particles to form between about 3 percent by weight to about 20 percent by weight dust particles based on the total weight of the alloy prior to mechanical abrasion.
U.S. Pat. No. 5,002,734 (Kowalczyk et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for the preparation of chalcogenide alloys which comprises crystallizing a chalcogenide alloy, grinding and pelletizing the crystallized product, and evaporating the alloy on, for example, a supporting substrate to form a photoreceptor.
Although known processes are suitable for their intended purposes, difficulties continue to be encountered in achieving precise control of tellurium and/or arsenic fractionation in the outer surface of a vacuum deposited photoconductive layer. This, in turn, affects the physical or electrical properties of the final photoreceptor. Photoreceptors containing large batch to batch top surface tellurium or arsenic concentrations tend to exhibit correspondingly large batch to batch variations in physical or electrical properties, which is unacceptable in high speed precision copiers, duplicators and printers because of copy quality variations. Moreover, variations in physical or electrical properties as a photoreceptor surface wears away during cycling is unacceptable in high speed precision copiers, duplicators, and printers, particularly during long length runs where, for example, the copy quality should be uniform from the first copy to thousands of copies. High speed copiers, duplicators and printers are constrained by narrow operating windows that require photoreceptors having precise, predictable operating characteristics from one batch to the next and during cycling.
Thus, there is a need for improved processes for preparing photoreceptors comprising selenium alloys containing additives such as tellurium and/or arsenic. There is also a need for processes for treating selenium alloys which control the relative quantity of tellurium and/or arsenic formed in the top surface layer of vacuum deposited selenium alloys containing tellurium and/or arsenic. A need further remains for processes for treating selenium alloys which maintain batch-to-batch top surface tellurium and/or arsenic concentrations in the top surface layer of vacuum deposited selenium alloys containing tellurium and/or arsenic. In addition, there is a need for processes for treating selenium alloys which reduce the tellurium and/or arsenic distribution variation through the thickness of a photoconductive layer of an alloy of selenium with tellurium and/or arsenic. Further, there is a need for processes for treating selenium alloys which limit the loss of selenium rich species early in the evaporation process. Additionally, there is a need for processes for treating selenium alloys which allow the achievement of TSA and TST values within narrower predefined limits. There is also a need for processes for treating selenium alloys which control the sensitivity of photoreceptors to light within narrower limits. A need also remains for processes for treating selenium alloys which produce evaporated films of selenium and its alloys with arsenic and/or tellurium which have superior photoconductive properties. In addition, there is a need for processes for treating selenium alloys which control the electrical cycling characteristics within narrower limits. Further, there is a need for processes for treating selenium alloys which control the mechanical wear characteristics of the photoreceptor surface within narrower limits. Additionally, there is a need for processes for treating selenium alloys which produce photoconducting devices which provide improved image quality when used in electrophotographic applications.